1. Field of the Invention
The present invention relates to a monolithic LC resonator and a monolithic LC filter, and more particularly, to a monolithic LC resonator and a monolithic LC filter for use in a high frequency wave band.
2. Description of the Related Art
FIGS. 13 and 14 show an example of a conventional monolithic LC resonator. As shown in FIG. 13, an LC resonator 100 includes a ceramic sheet 104 having a capacitor pattern 112 provided on the upper surface thereof, a ceramic sheet 105 having an inductor pattern 111 provided on the upper surface thereof, a ceramic sheet 106 having an input capacitor pattern 115 and an output capacitor pattern 116 provided on the upper surface thereof, ceramic sheets 102 and 108 having shield electrodes 113 and 114 provided on the upper surfaces thereof, respectively.
The ceramic sheets 101 to 108 are stacked, and fired to produce a laminate 110 shown in FIG. 14. On the laminate 110, an input terminal 121, an output terminal 122, and ground terminals 123 and 124 are provided. The input capacitor pattern 115 is connected to the input terminal 121. The output capacitor pattern 116 is connected to the output terminal 122. To the ground terminal 123, the lead-out portion of the inductor pattern 111, and one end of the shield electrodes 113 and 114 are connected. The lead-out portion of the capacitor pattern 112 to the ground terminal 124, and the other end of the shield electrodes 113 and 114 are connected.
In the above-described LC resonator 100, an inductor including the inductor pattern 111, and a capacitor including a capacitor pattern 112 opposed to the open end of the inductor pattern 111 define an LC parallel resonance circuit. The LC parallel resonance circuit is electrically connected to the input terminal 121 via a coupling capacitor including an inductor pattern 111 and the input capacitor pattern 115 opposed to each other. Similarly, the LC parallel resonance circuit is electrically connected to the output terminal 122 via a coupling capacitor including the inductor pattern 111 and the output capacitor pattern 116 opposed to each other.
The characteristics of the LC resonator depend on the Q value of the inductor in the resonance circuit. The Q value of the inductor is expressed as Q=2xcfx80f0L/R, in which L is the inductance of the inductor, R is the resistance of the inductor, and f0 is the resonance frequency. As seen in this formula, the Q value of the inductor can be increased by decreasing the resistance R of the inductor. The inductance R is inversely proportional to the cross section of the inductor pattern 111. Hence, the Q value is increased by increasing the cross section S of the inductor pattern 111.
However, where the thickness of the inductor pattern 111 is increased to increase the cross section S of the inductor pattern 111, the internal strain of the laminate 110 is substantially increased when the ceramic sheets 101 to 108 are integrally fired, resulting in delamination and other problems.
Further, a magnetic field generated in the periphery of the inductor pattern 111 is concentrated on the edge of the inductor pattern 111, causing a large eddy current loss. Moreover, in the conventional LC resonator 100, the magnetic field generated in the periphery of the inductor pattern 111 is interrupted by the capacitor pattern 112. Thus, the inductance L of the inductor is very low.
As described above, with the conventional LC resonator 100, it is difficult to attain a high Q value because the resistance R of the inductor pattern 111 constituting the LC resonance circuit is large, and moreover, the inductance L is low.
To overcome the above-described problems, preferred embodiments of the present invention provide a monolithic LC resonator and a monolithic LC filter each including an inductor having a high Q value.
According to a preferred embodiment of the present invention, a monolithic LC resonator includes a laminated body including an insulation layer, an inductor pattern, and a capacitor pattern laminated together, an LC resonance circuit in the laminated body that includes an inductor defined by the inductor pattern, and a capacitor defined such that the capacitor pattern is opposed to the inductor pattern with the insulation layer being sandwiched between the capacitor pattern and the inductor pattern. In the monolithic LC resonator, the inductor of the LC resonance circuit has a tubular structure defined by at least two inductor patterns electrically connected through via-holes provided in the insulation layer, and the capacitor pattern is located inside of the tubular structure of the inductor.
Further, according to another preferred embodiment of the present invention, a monolithic LC filter includes a laminated body including a plurality of insulation layers, a plurality of inductor patterns, and a plurality of capacitor patterns laminated together, a plurality of LC resonators in the laminated body which include the plurality of inductors defined by the inductor patterns, and the plurality of capacitors defined such that the capacitor patterns are opposed to the inductor patterns with the insulation layers being sandwiched between the capacitor patterns and the inductor patterns. In the monolithic LC filter, the inductor of each LC resonator has a tubular structure defined by at least two inductor patterns electrically connected through via-holes provided in the insulation layer, and at least one of the capacitor pattern and a coupling capacitor pattern for capacitance-coupling the LC resonators is located inside of the tubular structure of the inductor.
The surface area of the inductor having the plurality of the tubular structure can be increased without increasing the thickness of the inductor pattern. In general, high frequency current flows so as to be concentrated onto the surface of a conductor, due to the skin effect. Accordingly, the entire inductor, of which the surface area is increased; is effectively used as a path for high frequency current. Accordingly, the resistance of the inductor is greatly decreased as compared with that of a conventional inductor, and the Q value of the inductor is greatly increased.
Further, with high frequency current flowing through the inductor, a magnetic field is generated in the outer periphery of the tubular structure, not the inside of the tubular structure. Thus, the capacitor pattern and the coupling capacitor pattern for capacitance-coupling the resonators, arranged inside of the inductor having the tubular structure, do not substantially interrupt the magnetic field of the inductor.
Further, since the inductor has the tubular structure, the concentration of a magnetic field, generated in the periphery of the inductor, onto the edges of the inductor pattern is substantially reduced.
Other features, elements, characteristics and advantages of preferred embodiments of the present invention will become apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.